1. Field of the Invention
The present invention relates to an electron multiplier and a photomultiplier tube. More particularly, the present invention relates to an electron multiplier and a photomultiplier tube provided with a focusing electrode plate.
2. Description of Related Art
U.S. Pat. No. 5,504,386 discloses a photomultiplier tube of a multianode type. The photomultiplier tube includes a faceplate for receiving light. The faceplate is provided with a photocathode for converting the light into photoelectrons. A focusing electrode plate is located below the photocathode. A dynode unit and an anode unit are located in this order below the focusing electrode plate. The anode unit has a two-dimensionally arranged plurality of anodes.
The dynode unit is constructed from a plurality of dynode plates stacked one on another. The plurality of dynode plates include a first stage dynode plate that is located in the uppermost position of the dynode unit. Each dynode plate is formed with a plurality of channels. Each channel is constructed from one or more through-holes for multiplying incident electrons. It is noted that the plurality of channels are separated from one another with channel-separating portions. Each channel-separating portion has no through-holes, but has upper and lower surfaces.
In correspondence with the multi-channel structure, the focusing electrode plate is provided with a two-dimensionally arranged plurality of channel openings. That is, the focusing electrode plate is formed with a frame supporting a plurality of electrodes arranged in a grid pattern. The plurality of channels are separated from one another by the grid electrodes. Each grid electrode is located just above a corresponding channel-separating portion of the first stage dynode plate. Accordingly, the plurality of channel openings of the focusing electrode plate are located in confrontation with the plurality of channels of the first stage dynode plate. Each channel opening is for receiving electrons emitted from a corresponding position on the photocathode and for guiding the electrons to the corresponding channel in the dynode unit.
In the focusing electrode plate, an electric potential distribution is developed in each channel opening due to an electric potential of the grid electrodes surrounding the subject channel opening. The electric potential distribution guides the electrons from the corresponding position on the photocathode to the corresponding channel of the dynode unit. In the dynode channel, the electrons are successively multiplied and are finally collected at the corresponding anode. Thus, position-dependent detection can be attained on the light falling incident on the photocathode.